1. Field of Invention
The present invention relates to improvements in gaskets for automotive internal combustion engines. More particularly, the invention relates to MLS cylinder head gaskets having improved stoppers to provide enhanced sealing about combustion apertures of such gaskets.
2. Description of the Prior Art
Those skilled in the art will appreciate the difficulty involved in maintaining a high quality seal between cylinder apertures of an internal combustion engine block and a cylinder head fastened to the block. In recent years, multiple-layered steel (MLS) cylinder head gaskets have become a preferred design choice, wherein all (typically at least three) gasket layers have been formed of steel. Beaded exterior layers have generally been fabricated of 301 stainless steel, a relatively robust metal with a commensurately high spring rate, for meeting requisite performance requirements over the useful life of the gaskets. The center layer, also called a xe2x80x9cspacerxe2x80x9d layer, has generally been formed of less expensive metals such as 409 stainless steel, or in some cases even zinc-plated plain steel or other low carbon steels, for meeting less rigorous requirements.
The gasket areas immediately adjacent the circumference of engine cylinder bore apertures are subject to considerably greater stresses for assuring proper sealing than areas of the gasket radially remote from the apertures. To meet the greater stress requirements at the aperture areas, so-called stoppers have been employed in areas that circumferentially surround each cylinder aperture boundary. The stoppers are designed to provide increased sealing pressures around areas immediately adjacent the combustion apertures. In some cases the stoppers have been formed of extra layers of metal, consisting either of layers folded over or under primary sealing layers. In other cases, the stoppers have been formed as separately provided discrete annular rings positioned about the aperture boundaries.
Radially outwardly of the stopper, a secondary seal is normally provided. The secondary seal is usually formed as an active spring seal defined by an embossed bead in a primary external metal sealing layer. Two of such embossed axially spaced beads are normally provided in two separate mirror image external layers that sandwich a center or spacer layer. In some cases, the embossed beads employed in such duplicate mirror image primary sealing layers have been prone to cracking at the boundaries of the beads, particularly where metal stoppers have been employed. Stopper dynamics can create stresses on the beads that exceed normal bead deflection stress capability. Resultant cold working and bead cracking are seriously detrimental to both the performance and longevity of cylinder head gaskets capable of otherwise providing reliable combustion gas sealing media.
One commonly used type of stopper that existed before the advent of MLS gaskets is a wire ring stopper. Wire ring stopper structures have been recently applied to MLS gaskets, being formed to lie within the plane of, and radially juxtaposed against, a central unitary metal spacer layer. The combustion sealing beads in multi-layer steel gaskets have a tendency to crack in the vicinity of such wire ring stoppers, however, and may leak combustion gases as a result. Because the wire ring structures have had major difficulty withstanding stresses generated by thermally and dynamically induced vertical motions in MLS gaskets, MLS gaskets employing such wire ring structures have so far been associated with less than satisfactory performance results.
The disclosed invention provides a unitary, non-metallic circular stopper bead that circumferentially extends about the combustion apertures of an MLS combustion-sealing gasket. The circumferential wire ring offers a potentially more durable, as well as simpler, manufactured gasket.
In one described embodiment, a multi-layered steel cylinder head gasket incorporates the wire ring combustion seal. The gasket is comprised of inner and outer active sealing layers of full-hard stainless steel. Disposed between the inner and outer sealing layers is a low carbon or stainless steel spacer extending partially into a flange. A steel wire ring is disposed entirely within the flange; the flange is not contiguous with the inner and outer sealing layers. The gasket thus incorporates a gap between the flanged portion and the inner and outer active layers that helps to decouple stresses on the wire ring from those on the active layer. Separating the inner and outer active layers from the flanged portion of the gasket helps prevent cracking and enhance sealing. Depending on harshness of the application, either a low carbon or stainless steel wire ring may be employed, the stainless steel ring being preferred under the harshest of environments.
In summary, use of a steel wire ring combustion seal, spaced radially from the active gasket layers, helps to eliminate bead cracking. The wire ring facilitates conformability of the combustion seal, i.e. for sealing a greater range of engine hardware deflection. Localized motions are limited due to the relative robustness of the wire ring. Previous efforts to seal combustion chambers of heavy-duty diesel engines have been fraught with cracking and combustion seal failures. A flanged wire ring, situated in the manner described, is considerably more compliant to stresses applied both laterally and axially, and is thus resistant to cracking.
Finally, an embossed metallic secondary bead is formed radially outwardly of the already described primary stopper layer. The primary and secondary beads are designed to act in concert with each other, wherein the secondary sealing bead provides a backup in case of failure of the primary wire ring, as well as a seal targeted against leakage of engine oil or coolant into the combustion chambers of the engine.